Dual Emission with Efficient Phosphorescence Promoted by Intermolecular Halogen Interactions in Luminescent Tetranuclear Zinc(II) Clusters

The development of Zn-based phosphorescent materials, associated with a ligand-centered (LC) transition, is extremely limited. Herein, we demonstrated dual emissions including fluorescence and phosphorescence in luminescent tetranuclear Zn(II) clusters [Zn4LI4(μ3-OMe)2X2] (HLI = methyl-5-iode-3-methoxysalicylate; X = I, Br, Cl), incorporating iodine-substituted ligands. Single-crystal X-ray structural analyses and variable-temperature emission spectra studies revealed the presence of iodine substitutions, and intermolecular halogen interactions produced the internal/external heavy-atom effects and yielded strong green phosphorescence with a long emission lifetime (λmax = 510–522 nm, Φem = 0.28–0.47, τav = 0.78–0.95 ms, at 77 K). This work provided a new example that the introduction of halogen interactions is an advantageous approach for inducing phosphorescence in fluorescent metal complexes.


■ INTRODUCTION
−9 Thus, achieving a high phosphorescence quantum yield (QY), which is industrially important for Zn(II) complexes, remains challenging.−33 Although halogen bonding is commonly used to enhance the phosphorescence of organic molecules, there have been very few reports on the application of EHE through halogen bonding in metal complex systems.The further modification of fluorescent metal complexes using halogen bonding is an attractive avenue to develop pioneering multifunctional phosphorescent metal complexes.In our previous study, we demonstrated that a heptanuclear Zn(II) cluster [Zn 7 L 6 (μ 3 -OMe) 2 (μ 3 -OH) 4 ]I 2 (HL = methyl-3-methoxysalicylate), incorporating iodide counteranions, exhibited strong phosphorescence with an exceptionally long emission lifetime, which can be attributed to the EHE. 34This study offered a unique case of EHE-induced phosphorescence in metal complex systems, where halogen interactions acted as the trigger for the corresponding phosphorescence.This mechanism provides an efficient means of introducing SOC into luminescent metal complexes, particularly those associated with LC transitions.Consequently, extending the application of this system to similar types of multinuclear Zn(II) clusters is promising for the further development of multifunctional luminescent materials.Herein, we report novel tetranuclear Zn(II) clusters [Zn 4 L 4 I (μ 3 -OMe) 2 X 2 ] (HL I = methyl-5-iode-3-methoxysalicylate; X = I, Br, Cl), where the ligand HL I is iodine-substituted (Scheme 1).These Zn(II) clusters exhibit highly efficient phosphorescence and temperature-dependent emission color modulation, which are characteristics not observed in substitution-free Zn(II) clusters [Zn 4 L 4 (μ 3 -OMe) 2 X 2 ] (X = NCS, Cl, Br). 34EXPERIMENTAL SECTION Synthesis.All reagents and solvents were obtained from Tokyo Kasei Co. and Wako Pure Chemical Industries and were of reagent grade; they were used without further purification.All reactions were carried out under an ambient atmosphere.
Preparation of Tetranuclear Zinc(II) Complexes.Tetranuclear Zn(II) clusters were synthesized according to the method we described previously (with minor modifications). 36Zn 4 L 4 I (μ 3 -OMe) 2 I 2 ] (1).Triethylamine (0.202 g, 2.00 mmol) in methanol (10 mL) was added with stirring to methanol (20 mL) containing HL I (0.31 g, 1.00 mmol) and ZnI 2 (0.31 g, 1.00 mmol).The reaction mixture was stirred for 30 min at room temperature under air.The white microcrystals were participated and collected by suction filtration, washed with a small amount of methanol, and dried in air.Yield 78%.The single crystal suitable for the single X-ray structural analysis was obtained by allowing the mixed solution to stand for a few days to yield 1 as colorless block crystals.Anal.Calc.Physical Measurements. 1 H NMR spectrum for HL I was measured with a JEOL JNM-ECZS instrument.Elemental analyses (C, H, N, Cl, Br, and I) were performed on a J-Science Lab JM10 CHN analyzer.Infrared (IR) spectra measurements were performed on a HORIBA FT-730 instrument equipped with the KBr pellet method.Fast atom bombardment (FAB) mass spectra for 1 were measured with a JEOL JMS-AX505HA instrument with 3-nitrobenzyl alcohol (NBA) matrix.
Single-Crystal and Powder X-ray Diffraction.The singlecrystal X-ray diffraction data for 1−3 and HL I were recorded on a Bruker D8 QUEST diffractometer employing graphite monochromated Mo Kα radiation generated from a sealed tube (λ = 0.7107 Å).Data integration and reduction were undertaken with APEX3.Using Olex2 software, the structure was solved with the SHELXT structure solution program using Intrinsic-Phasing Methods and refined with the SHELXL refinement package using least squares minimization.Hydrogen atoms were included in idealized positions and refined using a riding model.Powder X-ray diffraction data (PXRD) for 1−3 were collected on a Rigaku MiniFlex II (40 kV/15 mA) X-ray diffractometer using Cu Kα radiation (λ = 1.5406Å) in the 2θ range of 5−30°with a step width of 3.0°.
Luminescence Property Measurements.Emission spectra at 298 and 77 K (Figures 2a,b and S5−S7) were measured using a JASCO FP-6600 spectrofluorometer.Emission quantum yields were recorded using a Hamamatsu Photonics C9920-02 absolute photoluminescence quantum yield measurement system equipped with an integrating sphere apparatus and 150 W CW xenon light source.The accuracy of the instrument was confirmed by the measurement of quantum yield of anthracene in ethanol solution (Φ = 0.27). 37,38mission lifetimes were recorded by using a Hamamatsu Photonics C4780 system equipped with a streak camera (Hamamatsu Photonics C4334) as a photodetector and a nitrogen laser (Usho KEN-X) for the 337 nm excitation.The emission decays of complexes were analyzed using two exponentials, i.e., I = A 1 exp(−t/τ 1 ) + A 2 exp(−t/ τ 2 ), where τ 1 and τ 2 denote the lifetimes, and A 1 and A 2 are the preexponential factors.Therefore, for the determination of radiative and

Inorganic Chemistry
nonradiative rate constants, the averaged emission lifetimes (τ av ) were estimated using the following equation: 39
■ RESULTS AND DISCUSSION Crystal Structures.HL I was prepared according to the method reported previously (Figure S1). 35The three tetranuclear Zn(II) clusters [Zn 4 L 4 I (μ 3 -OMe) 2 X 2 ] (X = I (1), Br (2), Cl (3)) were synthesized using our previously reported methods with minor modifications. 34,36Colorless crystals of 1−3 were obtained by allowing a mixed solution of the required Zn(II) salt, HL I , and triethylamine in methanol to stand at room temperature for a few days (Scheme 2).The obtained compounds were characterized by elemental analysis and single-crystal and powder X-ray diffraction (XRD) measurements.
Single-crystal X-ray structural analyses for 1−3 were carried out at 173 K. Individual structures of the tetranuclear Zn(II) clusters 1−3 are shown in Figures 1 and S2a, with the crystallographic data presented in Table S1.Each compound features a "defective" double-cubane core [Zn 4 O 6 ], where four Zn(II) ions are bridged by μ 2 -O atoms and μ 3 -methoxo groups from the deprotonated L Id − ligands (Figure S3b).One of the ligands L Id − is disordered.In 1, two distinct octahedral Zn(II) coordination spheres are present (Figure S3a).One sphere is formed by six oxygen atoms: two from bridging methoxo groups and four from bridging phenoxo group and carbonyl group on the L Id − ligands.The other sphere consists of an anion and four oxygen atoms: one from a bridging methoxo group and three from bridging phenoxo and methoxy groups on the L Id − ligands. 2 and 3 have structures identical to 1 (Figures S2  and S4).The tetranuclear structures of 1−3 are similar to those reported for Zn(II) clusters, which incorporate HL ligand derivatives. 34,40uminescence Properties.The solid-state excitation and emission spectra of the HL I ligand at 298 and 77 K are shown in Figures 2a and S5 (Table S2).At 298 K, the crystalline solid sample of HL I exhibited light blue emission.The emission spectrum exhibited a fluorescence band with a maxima (λ max ) at 473 nm attributed to the 1 ππ* transition.The solid-state excitation and emission spectra of compounds 1−3 at 298 and 77 K are presented in Figures 2b and S6, S7 (Table S2).At 298 K, crystalline solid samples of 1−3 displayed weak blue luminescence.The emission spectra for 1−3 featured broad, unstructured emission bands with maxima (λ max ) at 425, 438, and 425 nm, respectively.While slight variations in the emission maxima for 1−3 were noted, the overall emission profiles were similar.The observations described above for 1− 3 suggest that the emission maxima of 1−3 are largely independent of the coordinating anions.Importantly, in the spectra at 77 K, emission maxima of 510−522 nm were observed, which can be attributed to phosphorescence, alongside the emission bands at 425−438 nm.In addition, we evaluated the temperature dependence of the emission behavior at 300−100 K for 1 (Figure 2c,d).Although the emission intensities of the maxima increased slightly, the emission spectra remained almost unchanged below 250 K.Additional emission bands at 510 nm appeared from 200 K, corresponding to the emission-integrated intensity ratio of I phos./I fluo.(=I phosphorescence /I fluorescence ) increased to 1.3, which was almost 1:1, at 150 K (Figure 2e).Although the phosphorescence intensity increased significantly with decreasing temperature, the fluorescence intensity was almost saturated.The emission intensity ratios of I phos./I fluo.at 100 and 77 K were 12.5 and 66.2, respectively, indicating that the main contributor to the presented emission in the lowtemperature region was phosphorescence.In our previous work for the heptanuclear Zn(II) cluster [Zn 7 L 6 (μ 3 -OMe) 2 (μ 3 -OH) 4 ]I 2 , 34 the emission intensity ratio of I phos./I fluo.was 0.66 at 77 K, and no significant emission color change was observed even at 77 K (Figure S8).Therefore, efficient promotion of the formation of the triplet state by internal and/or external heavyatom effects via intermolecular halogen interactions is expected for 1−3.The emission images and CIE 1931 coordinates for the emission spectra of HL I and 1−3 clearly illustrate the color change of the emission from deep blue to green for 1−3, attributed to the expression of phosphorescence at low temperature (Figure 2b, inset, and Figure 2f), whereas the emission colors of HL I were almost unchanged even at 77 K (Figure 2a, inset).
Emission QYs (Φ em ) and emission lifetimes (τ) were measured for HL I and 1−3 (Tables 1 and S3, Figure S9).The emission QYs of HL I and 1−3 were identical (0.02) at 298 K. Due to the low emission QYs, the emission lifetimes at 298 K could not be detected.At 77 K, the emission QYs of 1, 2, and 3 clearly increased to 0.47, 0.31, and 0.28, respectively, whereas that of HL I was almost unchanged (0.03).The emission lifetimes (τ av ) of 1, 2, and 3 were in the millisecond range of 0.78, 0.91, and 0.95 ms, respectively.These values are consistent with the emission band attributed to phosphorescence involving the lowest-excitation triplet state (T 1 state).HL I exhibited a QY value at 0.03 that was significantly smaller than that found for the Zn(II) clusters (0.28−0.47).This difference in values is likely due to the ISC generated by the EHE of the intermolecular halogen interactions present in this case, as described below.The radiative decay rate constants (k r ) for 1−3 are 5.99 × 10 2 , 3.39 × 10 2 , and 2.95 × 10 2 s −1 , respectively.The order of magnitudes of these values may be reflected in the order of the heavy-atom effect for the coordinated halogen anions (I − > Br − > Cl − ).Although phosphorescence was not clearly observed at room temperature owing to thermal deactivation, the overall results highlight the significant contribution of the triplet excited states of 1−3, which are generated by the presence of iodine substitutions and EHE.The emission origin of the Zn-based phosphorescent molecular materials reported so far is predominantly associated with metal-to-ligand charge transfer (MLCT), 4 1 − 4 4 halogen-to-ligand charge transfer (XLCT), 45−47 and intraligand charge transfer (ILCT), 48 with extremely limited instances of phosphorescence reported to originate from LC transitions. 34,49Additionally, due to a scarcity of reports evaluating photophysical parameters, such as rate constants, the investigation of the estimated k r values in this study is highly important.Notably, the fact that these k r values for 1−3 are comparable to those reported for a Zn(II) complex exhibiting phosphorescence arising from Zn−Zn interactions (4.2 × 10 2 s −1 , at 77 K) 50 is noteworthy.
The observed differences in the photophysical properties of 1−3 and HL I can be attributed to the differences in their crystal structures and the resulting intermolecular interactions.The crystal packing diagrams of compounds 1−3 are shown in Figures 3 and S10 S4).Similar intermolecular interactions via halogen    4b).Based on the crystal structure analysis, halogen interactions occurred between the L Id − ligands of each molecule in 1−3, whereas no halogen-related interactions occurred for HL I .This is a key factor in the observed phosphorescence differences between 1−3 and HL I in the solid state.
To evaluate the contribution of the internal heavy-atom effect (IHE) and EHE, we investigated the variable-temperature emission spectrum for 1 in solution (MeOH, 1.0 × 10 −5 M) (Figure 5).At 300−200 K, 1 exhibited a weak blue emission with an emission maximum at 425 nm.This is because nonradiative transitions reflect the violent molecular vibrations of 1 in solution.At 150 K, the emission intensity at 425 nm increased drastically (approximately 9 times).This can be attributed to the inhibition of molecular vibrations induced by the frozen MeOH solution.However, phosphorescence at   Inorganic Chemistry 510 nm was not clearly observed at 150 K. Upon further decreasing the temperature, the emission bands remarkably appeared at 510 nm, which can be attributed to the phosphorescence.The emission-integrated intensity ratio of I phos./I fluo.at 150 and 77 K was 0.2 and 2.6, respectively (Figure 2e).Compared with that in the solid state, phosphorescence in the solution state decreased, which indicates the key role played by EHE, rather than IHE, induced by intermolecular halogen interactions.To reveal the luminescent species, FAB mass spectrum (MS) analysis was performed for 1 in a MeOH solution.The observed spectrum showed dominant peaks at m/z = 1267.53,attributable to the heptanuclear species of {[Zn 7 L 6 I (μ 3 -OMe)(μ 3 -OH) 5 ]I} 2+ , rather than a tetranuclear species.This indicates the possibility that dissolution in MeOH or ionization by MS leads to structural conversion to the heptanuclear species.However, it should be noted that these results do not lead to a precise identification of the correct luminescent species.On the other hand, the results of variabletemperature emission spectra for HL I in MeOH solution showed clearly different behaviors from those of 1 (Figure S14).At 77 K, HL I exhibited a structured fluorescence band at 405 nm and a broad phosphorescence band at 510 nm, both with similar intensity.Therefore, the difference in luminescence behavior between the solid state and solution of 1 can still be attributed to EHE due to intermolecular halogen interactions.The CIE 1931 coordinates for the temperature dependence of the emission spectra of 1 clearly demonstrate the emission color differences and modulations from deep blue to green, attributed to the expression of phosphorescence in both the solid state and the solution (Figure 6).The above unique photophysical properties observed in 1−3 were not observed in the substitution-free Zn(II) clusters. 34,40

CONCLUSIONS
In conclusion, we synthesized tetranuclear luminescent Zn(II) clusters [Zn 4 L 4 I (μ 3 -OMe) 2 X 2 ] (X = I, Br, Cl) and demonstrated that the intermolecular halogen interactions produced the EHE for the expression of strong phosphorescence with long emission lifetime.An improvement over previous studies is noteworthy, as the observed temperature for clear phosphorescence has increased to around 200 K.Further improvement for the development of novel Zn(II) complexes exhibiting room-temperature phosphorescence is currently in progress.Importantly, the formation of halogen interactions by introducing halogen substitutions is an advantageous approach, which can result in highly efficient phosphorescence and functions related to the dual emissions 51,52 in both organic molecules and fluorescent metal complexes.This result also suggests the possibility of achieving luminescence switching induced by structural rearrangements triggered by other external stimuli, such as mechanical force, and the development of novel optofunctional molecular systems.

Figure 2 .
Figure 2. Emission spectra for (a) HL I and (b) 1 in the solid state at 298 K (black) and 77 K (red) (inset: emission images under UV light (365 nm)).(c, d) Temperature dependence of emission spectra for 1 at 300−100 K. (e) Emission-integrated intensity ratio (I phosphorescence /I fluorescence ) for 1 at 200−77 K in the solid state (red) and MeOH solution (blue) (inset: enlarged figure).(f) CIE 1931 chromaticity diagrams for emission spectra of HL I and 1−3 in the solid state at 298 and 77 K.

Figure 3 .
Figure 3. Packing structure of 1 at 173 K.All disordered atoms have been omitted for the sake of clarity.Blue dashed lines represent halogenrelated interactions (CH−I, I−I, and I−π interactions).

Figure 4 .
Figure 4. (a) Dimeric structure of HL I at 173 K. (b) Crystal packing structure of HL I at 173 K. Blue dashed lines represent π−π interactions, and orange-dashed lines represent CH−O interactions between each dimer.

Figure 6 .
Figure 6.CIE 1931 chromaticity diagrams for the temperature dependence of emission spectra of 1 at 300−77 K in (a) the solid state and (b) MeOH solution.

Table 1 .
Photophysical Properties for HL I and 1−3 in the Solid State at 77 K CCDC 2332303−2332306 contain the supporting crystallographic data for this paper.These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.